Stretched films of polyvinyl chloride, polypropylene, polyethylene and the like are hitherto known as heat shrinkable films. Among them, heat shrinkable polyethylene films have been put to practical use from the viewpoints of possession of heat sealability and low cost. Particularly, in recent years, a heat shrinkable polyethylene film using a linear low density copolymer of ethylene and an .alpha.-olefin (hereinafter referred to as "linear low density polyethylene") attracts attention in that the impact resistance and the heat seal strength are excellent, and its utilization in various fields is expected.
The present inventors previously proposed a heat shrinkable film composed mainly of a specific ethylene/.alpha.-olefin copolymer (Japanese Patent Publication Kokai No. 62-201229). By practicing this proposed method, it became possible to obtain a film having a less unevenness in thickness and having a better low temperature shrinkability than films obtained by inflation. However, in case of application to automatic packaging machines (such as pillow type packaging machine and automatic packaging machine using centerfold films), the proposed film encounters a problem that a heat seal failure (non-sealing in part) which has not been generated, is generated because the packaging speed of packaging machines is markedly increased in recent years. When packaging is made by automatic packaging machines, it is general to conduct the heat sealing by bead sealing using a heat knife. The sealing failure phenomenon means that a sealed portion is peeled off in a shrinking step, a film is not well cut off with causing stringiness at the sealed portion when a large tension is applied to the film (for example, when bulky goods are packaged), pinholes are generated at the sealed portion, or in an extreme case a film is not sealed at all.
The sealing failure as mentioned above also occurs when a molten resin adheres to the edge of a heat knife or to a heat knife pedestal.